CK-12 Physical Science - For Middle School

http://www.ck12.org Chapter 11. Nuclear Chemistry

14 days, half of the original amount of phosphorus-32 has decayed. After another 14 days, half of the remaining
amount (or one-quarter of the original amount) has decayed, and so on.

FIGURE 11.9

This diagram models the rate of decay of

phosphorus-32 to sulfur-32.

Different radioactive isotopes vary greatly in their rate of decay. As you can see from the examples inTable11.1,
the half-life of a radioisotope can be as short as a split second or as long as several billion years. You can simulate
radioactive decay of radioisotopes with different half-lives at the URL below.

http://www.colorado.edu/physics/2000/isotopes/radioactive_decay3.html

Some radioisotopes decay much more quickly than others.

TABLE11.1:Half-Life of Some Radioisotopes

Isotope Half-life

Uranium-238 4.47 billion years

Potassium-40 1.28 billion years

Carbon-14 5,730 years

Hydrogen-3 12.3 years

Radon-222 3.82 days

Polonium-214 0.00016 seconds

Problem Solving

Problem: If you had a gram of carbon-14, how many years would it take for radioactive decay to reduce it to
one-quarter of a gram?

Solution:One gram would decay to one-quarter of a gram in 2 half-lives

( 1

2 ×

1

2 =

1

4

)

, or 2×5,730 years = 11,460
years.

You Try It!

Problem:What fraction of a given amount of hydrogen-3 would be left after 36.9 years of decay?

Radioactive Dating

Radioactive isotopes can be used to estimate the ages of fossils and rocks. The method is calledradioactive dating.
Carbon-14 dating is an example of radioactive dating. It is illustrated in the video at this URL: http://www.youtu
be.com/watch?v=udkQwW6aLik (11:00).